A process for selectively metallizing a circuit board is described in U.S. Pat. No. 5,162,144, (assigned to the same assignee as the present application) and hereby incorporated by reference. This process is used in conjunction with a two dielectric layered, sequentially processed circuit board, as described in U.S. Pat. No. 5,260,170 (assigned to the same assignee as the present application). This circuit board process consists of the sequential deposition and photo-definition of openings in two resins. The first resin, called resin A, is filled with a material referred to herein as a filler, which when activated, will promote electroless plating with a metal. The second resin, called resin B, does not contain such a filler. When each resin, in turn, is photochemically patterned, resin B over resin A, openings produced in resin B define areas called "channels"; superimposed openings produced in resin A and resin B define deeper areas called "wells" which open to a metal layer below. When this structure is subjected to processes which modify or activate particles contained in or at the surface of resin A, the channels and wells may be coated with an electroless plated metal to form metallic features, such as pads, vias and conductors.
In building the circuit board it is important that the electroless plated metal stay confined to the defined pads, vias and conductors. Otherwise the circuit board will contain shorts and other undesirable properties. One process for activating the particles at the surface of resin A involved immersing the circuit board in a reducing agent of borohydride. In the preferred case, the particles in resin A are Cu.sub.2 O particles at a concentration of less than 10% by weight of dry resin. When the Cu.sub.2 O particles are exposed to the borohydride, the Cu.sub.2 O particles are converted to Cu or CuH. When the copper is in this active state (Cu, CuH) it is catalytic to electroless plating. Given the limited amount of particles in resin A there is an insufficient number of catalytic sites for the electroless plating to form continuous lines, as required to form conductors.
For continuous production applications where circuit boards are carried by a conveyor belt it is necessary to be able to apply the reducing agent in a suitable manner, i.e. spraying. Spraying has the advantageous effect of spreading the active copper away from the particle sites. This spreading allows the formation of continuous conductors. However, the spreading is not confined to the exposed resin A areas. As a result, bridging occurs when the spraying operation is not carefully applied to the defined pads, vias and conductors. The controlled spraying works in the laboratory but is impractical for the production environment. As a result, there exists a need for a method and a reducing solution that forms large catalytic islands on resin A without the catalytic islands forming on resin B to produce finely space conductors, vias and pads.